CHASSAHOWITZKA  Bird lovers trying to establish a migrating colony of whooping cranes east of the Mississippi River say they will dedicate at least three to five more years to the project.

That's how long it will take to reach their goal of having 125 cranes, including 25 breeding pairs, in the colony, said Heather Ray, of Operation Migration.

Her group is joined by the International Crane Foundation, the U.S. Fish & Wildlife Service and state agencies such as Florida's Department of Environmental Regulation in annual, human-guided flights that may help get the birds off the endangered species list.

"The partnership has been remarkable considering all of the agencies and nonprofits involved," said Joan Garland, of the crane foundation. "We've really been clicking."

Once relatively common in North America, whooping cranes were nearly extinct by 1941, when only about 20 remained in the wild.

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They've rebuilt their numbers slowly, including 270 in a flock that migrates from Canada to the Texas Gulf Coast.

There also is a 100-bird nonmigrating colony near Kissimmee.

The eastern experiment began in 2001 when seven fledglings were taught to identify ultralights as surrogate mothers. The tiny planes emit a sound similar to a whooping crane mom, and the pilots dress in costumes that look like cranes.

Fifty-three young birds have made the human-led flights from their summer home at Necedah National Wildlife Refuge in rural Wisconsin to their wintering grounds at Chassahowitzka National Wildlife Refuge on the Citrus-Hernando county line.

The 1,250-mile journey has taken from 48 days in 2001 to 64 days in 2004, their slowest pace thanks to bad weather.

Six of the original 53 have been killed by predators, including two by bobcats their first year at Chassahowitzka.

Since then, the pens that serve as their home have been stabilized against predators.

This year's 13 whoopers have been joined at Chassahowitzka by three earlier graduates.

"We don't want the others here," Ray said. "We want them to winter where they want, whether it's Florida or South Georgia. That will help strengthen the colony. If there are too many adult birds [at Chassahowitzka], they pick on the little ones."

In the fall, the young birds return north by following their elders or their cousins, sandhill cranes. Getting the colony to breeding age means having birds that are at least 5 years old. "So we're looking at another year before we begin having breeding pairs," Garland said.

This year the team hopes to add a new wrinkle.

"We hope to begin supplemental releases" such as what happened this year with a fledgling called No. 18, Garland said.

Its feathers didn't develop early enough to fly with its 13 ultralight-led friends. So it was released with and followed older birds.

"It's doing well at another location in Florida," Garland said, not revealing where for fear of human intrusion.